Integrated circuitry may be fabricated over and within semiconductor substrates. Individual device components of the circuitry may be separated or electrically isolated from other components by dielectric or other isolation formed over and/or within the semiconductor substrate. One form of isolation is commonly referred to as trench isolation, wherein trenches are etched into semiconductor substrate material and subsequently filled with one or more dielectric materials. The trenches and isolation material therein may surround islands of semiconductor substrate material, commonly referred to as active area regions, where some of the individual circuit components are fabricated.
Integrated circuitry can be fabricated to have many functions, and may include many different electronic devices such as capacitors, transistors, resistors, diodes, etc. One type of transistor is a field effect transistor that has its gate construction received wholly or partially within a trench formed in semiconductor material of the active area regions. A gate insulator separates the conductive gate from the semiconductor material. A pair of source/drain regions is formed within semiconductor material elevationally higher than the conductive gate material. Application of suitable voltage to the conductive gate material within the trench enables current to flow through the semiconductor material between the source/drain regions along the trench sidewalls and around the base of the trench. The gates of buried transistors in some circuit constructions transversally cross some active area regions and extend between other immediately end-to-end adjacent active area regions.
A continuing goal in integrated circuitry fabrication is to form smaller individual components and to maximize circuit density by placing those components closer together. Accordingly, the trenches within which buried transistor gate lines are received are becoming narrower as are the lateral thicknesses of the gate insulator and conductive gate material within those trenches. Multiple different composition conductive materials are commonly used for the transistor gates to facilitate deposition, serve as diffusion barriers, and/or impart desired work-function. These different composition conductive materials typically have different respective electrical conductivity/resistivity. It is desirable to maximize volume of the higher conductive material(s) versus the lower conductive material(s) to maximize conductivity of the buried transistor gate lines, particularly as the overall volume of all conductive material of such gate lines decreases.